1. Field of the Invention
The invention relates to a vacuum filter belt apparatus.
2. Description of the Prior Art
Typically, such apparatus has an endless belt of elastic material with a concave transverse section, guiding means provided with a drive system for moving the belt along a path having a horizontal operative path portion, fixed supports for supporting and guiding the belt along the operative path portion and a vacuum chamber extending beneath the middle of the belt along operative path portion. Passages in a thick or enlarged portion along the middle of the belt connect via a central slot in the upper surface of the vacuum chamber to the vacuum chamber, so that suction is applied to the upper surface of the belt and the belt makes an airtight seal against the vacuum chamber.
Vacuum filters are widely used industrially for washing or lixiviating slurry mixtures. A typical example of this is the production of H.sub.3 PO.sub.4 from a mixture of phosphate-bearing rock and sulphuric acid. The slurry formed from this reaction consists of gypsum and phosphoric acid, with the phosphoric acid being concentrated and separated from the gypsum by means of the vacuum filter. This is a procedure which is known and need not be explained further, and which is moreover mentioned only as one possible application of vacuum filter installations in general. Vacuum filter belts are themselves an improvement on the so-called pan filters which consist of a series of separate pans each of which is connected to a vacuum installation and which are conveyed around a horizontal circular track like a caroussel. The pans, are fitted with a filter and, in turn, are filled, have suction applied to them, are flushed and are emptied by tilting.
Compared with pan filters, the vacuum filter belt is an improvement since it does not necessitate periodic tilting movements of the pans and consequently runs more regularly and is simpler to design. Further, the filter is more accessible and the undesirable consequences of deposits within the filter are also less serious. The elastic belt is provided on its upper side with ribs and grooves on which a filter cloth can be laid. This filter cloth, which is also continuous, rests in the operative path portion of the belt on the ribs on the upper surface of the belt but is moved round separately outside this path portion allowing the filtered cloth to be easily cleaned and any solid deposits to be removed from the belt.
The belt's own weight presses it against the fixed supports so that the belt is guided along a fixed path. In known vacuum filter belts, the belt lies over the full width of its thick central portion on a fixed flange on the upper side of the vacuum chamber. The total force with which the belt is pressed against the vacuum chamber is proportional to the degree of the vacuum within the chamber, the width of the thick portion of the belt and the length of the chamber. This means that the belt is forcefully sucked against the vacuum chamber with considerable force. Even if materials with a low coefficient of friction are used, the total friction between the belt and the vacuum chamber still requires an appreciable force to be used to drive the belt. On the one hand, this means that an expensive drive system is necessary, and on the other hand that the belt itself must be of an especially heavy and strong design for this purpose. It should be noted that the friction between the belt and the guiding supports also contributes substantially to the total traction force required for the belt, but it has been found that this may be reduced by applying liquid or air lubrication between the belt and the supports. Since the contact surface between the thick portion of the belt and the vacuum chamber must also serve as an airtight seal, it has in fact proved difficult to use water or air lubrication in this case.